Glossary · Thiên văn học
Precession
The slow change in direction of a spinning object's rotation axis under a torque, like the wobbling axis of a spinning top. For Earth, it is the very slow change in the direction of the axis relative to the stars, which changes the North Star over thousands of years.
In mechanics, when a torque acts on a spinning object, its rotation axis, which points along the angular momentum, can slowly sweep around another axis instead of toppling over. This is precession, clearly visible in a gyroscope.
Earth bulges slightly at the equator. The gravitational pull of the Sun and Moon on this bulge makes Earth's axis precess, completing one circuit in about 26,000 years (NASA gives about 25,771.5 years). This is axial precession; it is separate from changes in the tilt of the axis, which follow a cycle of their own.
Because the axis changes direction, the bright star near the north celestial pole changes over time: today it is Polaris, a couple of thousand years ago it was Kochab, and in about 12,000 years it will be Vega. Precession also makes seasonal contrasts stronger in one hemisphere and weaker in the other, and it is one of the Milankovitch orbital cycles that affect Earth's climate over very long timescales.